All Sprague-Dawley (S-D) rats, fed dimethylhydrazine (DMH) at dose level of 30 mg/kg body weight/week/5 weeks developed intestinal tumors at 20 weeks after first dose of DMH. A dose response was demonstrable in DMH-treated Lobund S-D rats. Rats on the 5 dose DMH protocol and then fed Na barbiturate in the drinking water (0.1%) developed increased numbers of intestinal tumors compared to the control rats on drug-free water. Rats on the DMH protocol, and then fed indomethacin in the drinking water, developed fewer tumors, and 40% of them were tumor-free.